1. Field of the Invention
This invention relates to drag type rotary drill bits especially adapted for use in drilling oil and gas wells in earth formations, and, more particularly, this invention relates to drag type rotary drill bits using individual cutters arranged over the face of the bits including bits known as polycrystalline diamond compact bits, referred to herein as "PDC drill bits".
2. Description of the Prior Art
Drag type rotary drill bits have been known, particularly in the oil and gas industry, for a substantial number of years. They are connected on the lower end of an assembly of pipe sections secured in end to end array and known as "drill string". The drill string is rotated to turn the bit while advancing the bit downwardly to disintegrate or gouge out portions of an earth formation as the cutters are forced into the formation in a downwardly spiraling pattern. As is also well known, such bits have included flow channels for directing drilling fluids from the drill string outwardly through the bit around the cutters for keeping the bit cool, flushing cuttings from around the bit upwardly in the annulus around the drill string, and for imposing a hydrostatic head on the formation being drilled to retain formation fluids in the formation as a well is drilled. In recent years, cutting elements used in drag type rotary bits have been formed of super hard materials, such as tungsten carbide, and also of especially effective and more commonly used materials known as "thermally stable polycrystalline diamond material". The artificial diamond material may be used individually or as a component of a composite compact or insert on a cemented tungsten carbide substrate. Recently, a new artificial polycrystalline diamond has been developed which is stable at higher temperatures than previously known polycrystalline diamonds. Both types of polycrystalline diamonds are available in a wide variety of shapes and sizes. Polycrystalline diamond composite compacts are commercially available to the drilling industry from General Electric Co. under the "STRATAPAX" trademark. A comprehensive description of STRATAPAX diamond cutting elements and prior art bits utilizing the elements is found at pages 541-591 of a book entitled ADVANCED DRILLING TECHNIQUES by William C. Maurer, published by The Petroleum Publishing Company, 421 S. Sheridan, P.O. Box 1260, Tulsa, Oklahoma 74101.
PDC drill bits have been manufactured utilizing a wide variety of techniques and structures for mounting the cutters over the face of the bit and an equally wide variety of patterns or position arrangements of the cutters. In some prior art bits the cutters are individually mounted in various patterns. In other forms of prior art bits the cutters are mounted in group or clusters arranged in a wide variety of patterns. U.S. Pat. No. 4,073,354, to Rowley et al, issued Feb. 14, 1978, shows individual polycrystalline diamond cutters arranged over the face of a bit extending from the bit center to the bit gage in longitudinal arrays. Additionally, the center of the bit includes small individual diamonds set between the cutters to supplement the cutting effect of the cutters at the bit center. Other similar arrangements of the use of individual separate cutters are shown in U.S. Pat. Nos. 4,700,790, issued Oct. 20, 1987 to Shirley, 4,733,735 issued Mar. 29, 1988 to Barr et al, 4,596,296, issued June 24, 1986 to Matthias, 4,323,130, issued Apr. 6, 1982 to Dennis, UK patent application GB 2,085,945 of Jurgens published May 6, 1982 and in U.S. Pat. No. 4,350,215 issued Sept. 21, 1982 to Radtke, the latter patent showing cutters arranged in a longitudinally extending spiral pattern from the face of the bit to the bit gage. U.S. Pat. No. 4,696,354 issued Sept. 29, 1987 to King et al, shows individual cutters arranged in longitudinal radial array from the center face of the bit to planar pads along the bit gage portion. Small diamonds are embedded in the planar pad surfaces interrupted by longitudinal troughs. A similar bit using diamond cutters mounted individually in longitudinal radial alignment along the bit face is shown in U.S. Pat. No. 4,733,734 issued Mar. 29, 1988 to Bardin et al.
Further bit designs using diamond cutters arranged in clusters or groups are shown in U.S. Pat. No. 4,667,756 issued May 26, 1987 to King et al and in U.S. Pat. No. 4,714,120, issued Dec. 22, 1987 to King. These patented bits use clusters of tightly grouped cutters which share a common backing. The numbers of cutters included in each cluster vary. In the bit of U.S. Pat. No. 4,714,120, individual cutters are mounted on the bit face as well as clusters each including as few as two cutters and others as many as four cutters. A still further form of bit using thermally stable polycrystalline diamond material is shown in U.S. Pat. No. 4,602,691 issued July 29, 1986 to Weaver. The Weaver bit uses a variety of shapes of cutting elements grouped in like kinds extending in longitudinal radial rows from the bit center to the bit gage which is set with small diamond particles. Along the junction between the bit face and the gage of the Weaver bit, some rows include cylindrical shaped diamond inserts. So far as is presently known, those prior art bits previously described, as well as other known available bits, using cylindrical cutters and cutters having faces in the shapes of segments of circles utilize cutters of substantially the same area which generally range from 0.50 inches to 0.58 inches in diameter. Recently, cutters ranging from 0.70 to 0.79 inches in diameter and 0.90 to 1.0 inches in diameter and even larger have been available to the bit industry. The bit designs using these larger diameter cutters have set the entire bit with cutters of the same diameter, or, alternatively, have reduced the cutter diameter only at the very gage of the bit. In some instances smaller diameter cutters in the 0.50 inch range have been used at the centers of bits and then intermixed across the bit face with larger diameter cutters. In still other bits, cutters arranged over most of the bit face have been 0.50 inches in diameter with larger cutters set only on the bit shoulder. In drilling bore holes with the available prior art PDC bits, particularly in fast drilling applications which include transitional layers of shale and sand, those bits employing larger cutters tend to fail on the bit shoulder. Bits utilizing small cutters tend to slow down in the shale. Additionally, where the cutters are individually mounted, the cutters are more likely to fail by shearing from the bit matrix.